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Last active November 13, 2016 14:46
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Safe web routes in PureScript
module SafeWeb where
import Prelude
import Data.Array (filter)
import Data.Leibniz (type (~))
import Data.String (Pattern(Pattern), split, joinWith)
type Path = Array String
pathToHtml :: Path -> String
pathToHtml = (<>) "/" <<< joinWith "/"
pathFromString :: String -> Path
pathFromString = filter ((/=) "") <<< split (Pattern "/")
data GET = GET
data POST = POST
data Route m = Route m Path
class Requestable r where
toRoute :: forall m. r m -> Route m
type WithMethod m r = m -> (m ~ m) -> r m
a :: forall r. Requestable r =>
WithMethod GET r
-> String
a r =
case toRoute (r GET id) of
Route GET path ->
"<a href=\""
<> pathToHtml path
<> "\"></a>"
form :: forall r. Requestable r =>
WithMethod POST r
-> String
form r =
case toRoute (r POST id) of
Route POST path ->
"<form method=\"post\" action=\""
<> pathToHtml path
<> "\"></form>"
module SafeWebSpec where
import Prelude
import SafeWebSpec
import Data.Leibniz (type (~))
import SafeWeb (form, POST, GET, a, Route(Route), class Requestable)
import Test.Spec (Spec, it, describe)
import Test.Spec.Assertions (shouldEqual)
type Id = Int
type Project = { name :: String }
-- Here we create the routes in our application, and restrain
-- them to specific HTTP methods, using the Leibniz type equality
-- operator `~`. The framework takes care of providing `m` and the
-- proof `m ~ <METHOD>`.
data MyRoutes m
= Home m (m ~ GET)
| GetProjects Id m (m ~ GET)
| SaveProject Project m (m ~ POST)
-- This part would be very nice to derive generically.
instance requestableMyRoutes :: Requestable MyRoutes where
toRoute r =
case r of
Home m _ ->
Route m ["about"]
GetProjects id m _ ->
Route m ["projects"]
SaveProject project m _ ->
Route m ["projects"]
spec :: forall e. Spec e Unit
spec = do
describe "a" do
it "produces an anchor tag for a GETable resource" do
-- Here we create an anchor tag to GetProjects. We cannot pass
-- this value to `form`.
a (GetProjects 1) `shouldEqual` "<a href=\"/projects\"></a>"
describe "form" do
it "produces an form tag for a POSTable resource" do
let project = { name : "SafeWeb" }
-- Same safety here, we can pass `SafeProject project` to `form`, but not
-- to `a`.
form (SaveProject project)
"<form method=\"post\" action=\"/projects\"></form>"
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owickstrom commented Nov 12, 2016

Things to consider:

  • We are never using the proof in the Leibniz value, just demanding that it exists.
  • The Requestable instance must be hand-written, and it's quite repetitive. Could perhaps be generically derived. The question there is how to handle the paths automatically?

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